Hi Marie Claire,
Localization of RadRichTextBox is fully supported, although this is not well documented for now. You could refer to the following article about localizing RadControls in general. As RadRichTextBox doesn't have detailed description of its resource keys, I have prepared you sample resource file (find attached) that you can use for your translation. The keys for context menu's commands have prefix "Documents_ContextMenu".
Don't hesitate to contact us if you have other questions.
the Telerik team
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